Pulmonary hypertension (PH) is a syndrome which the World Health Organization subdivides into five classes, including Group I pulmonary arterial hypertension (PAH), which is PH due to pathology of the precapillary pulmonary vasculature, and Group III, which is PH due to chronic hypoxemia. The pathogenesis of PAH remains poorly understood and, despite advances in therapy, mortality due to PAH remains high. Our laboratory has identified a novel signaling pathway which is necessary for the development of hypoxia- induced PH in mice. In this pathway, hypoxia induces activation of the transcription factor HIF-1 in pulmonary artery smooth muscle cells (PASMCs); HIF-1 in turn upregulates expression of the transmembrane ion exchanger NHE1, which effects changes in PASMC migration and proliferation that contribute to the pathologic pulmonary vascular remodeling that leads to PH. Interestingly, HIF-1 expression has been found to be increased in normoxic PASMCs from human patients with PAH, in comparison with normal controls. Furthermore, our preliminary studies show that NHE1 activity is increased in PASMCs in PAH models compared to controls. Therefore, the focus of this proposal is to determine the role of the NHE1 signaling pathway in the development of PAH as well as to test the efficacy of NHE1 inhibitors in the treatment of PAH. The long-term goal is that, by elucidating new pathways that contribute to the pathogenesis of PAH, we may discover novel targets for therapy. We will employ two models of PAH: an in vitro model comparing PASMCs from PAH patients to those from normal patients, and an in vivo model comparing rats, in which PAH is induced through the combination of Sugen exposure and hypoxia, to normoxic rats. In Specific Aim 1, we will determine whether NHE1 expression is increased in PAH models and, if so, whether NHE1 upregulation in PAH is dependent upon HIF-1. Through overexpression as well as silencing of NHE1, we will determine whether NHE1 levels in PASMCs contribute to pathologic PASMC proliferation and migration. In Specific Aim 2, we will determine the efficacy of cariporide, a specific NHE1 inhibitor, in the prevention and reversal of PAH in our rat model, as well as the effect of cariporide on human PASMC function in our in vitro model. In addition to determining the role of NHE1 in the pathogenesis of PAH, execution of this proposal will provide training for the candidate that will be instrumental toward his career goal of becoming a physician- scientist performing basic science research in the field of pulmonary medicine.